PPS has excellent heat resistance, chemical resistance, and electric insulation and, also, excellent mechanical strength and molding workability. Accordingly, PPS is widely used as a metal substitute material and a material capable of enduring extreme environment. Various applications are also proposed for PPS fibers in view of these properties, and exemplary applications include filter, bristle of brush, canvas of dryer in paper making, electric insulation paper and other industrial materials. For example, Japanese Unexamined Patent Publication (Kokai) No. H10-60734 discloses a method of producing a PPS fiber with reduced fineness irregularity in a stable manner.
Recently, use of the PPS monofilament has been investigated as a substitute for SUS steel wire in the production sites of the fields including chemistry, electrics and electronics, automobiles, foods, precision machines, and pharmaceutical products and medicine. For example, Japanese Unexamined Patent Publication (Kokai) No. 2012-246599 proposes a method wherein a multifilament is first produced and then separated into monofilaments for the production of the monofilament with high productively at low cost. In that method, however, stretching and thermal setting are conducted with the monofilaments entangled by the entanglement treatment in step of producing the multifilament and the stretching and thermal setting are likely to be inconsistent between respective monofilaments, and the monofilaments after the separation are likely to suffer from fineness irregularity in axial direction of the fiber detracting from the production of consistent monofilaments. In addition, many guides and similar members are present in the separating steps, and the filaments are easily damaged by the friction and, because of the frictional tension and the like, the monofilament after the separation exhibited increased variation of the continuous heat shrinkage stress in axial direction of the fiber detracting from the production of consistent monofilament. Accordingly, the consistency in the physical properties in axial direction of the fiber was lost in the separated filament, and such separated filament was incapable of producing the high precision filters.
Furthermore, Japanese Unexamined Patent Publication (Kokai) No. 2011-106060 proposes a method wherein a small amount of a polyalkylene terephthalate is added to obtain the PPS monofilament having improved dimensional stability. However, due to addition of the polyalkylene terephthalate, that method suffered from the problems of loss of heat resistance and chemical resistance as well as loss of consistency in the physical properties in longitudinal direction by the blend irregularity, which resulted in the incapability of use for the high precision filter.
Furthermore, Japanese Unexamined Patent Publication (Kokai) No. 2009-68149 proposed a PPS monofilament having improved consistency of fineness in the intention of improving filter performance. While that PPS monofilament was certainly capable of producing filters having a certain precision, it was found that that PPS monofilament was incapable of responding to the recent demand for drastic increase in filter precision because an increase in filter precision requires control of the variety in the tension during the weaving of the filter, and Japanese Unexamined Patent Publication (Kokai) No. 2009-68149 does not at all indicate the method of solving such problems. As described above, there has been no PPS monofilament adapted for use in producing a high precision filter having an extremely low aperture variation rate and, accordingly, there is a strong demand for a PPS monofilament adapted for use in high precision filter.
It could therefore be helpful to provide a PPS monofilament with a very low aperture variation rate that is highly adapted for use in high precision filters.